Pulsed laser drilling is used to produce bore holes having small diameters, for example, in hollow workpieces. Turbine blades, in particular, have a multitude of fine cooling air bore holes, which this method is able to produce with high positional accuracy and in an automated manner.
However, in order to achieve the cooling air flow rate required during operation these bore holes have to conform to exact tolerances with regard to their diameter. For that reason, the dimensional accuracy of the produced bore holes must be checked.
Furthermore, it should be ensured, for one, that the bore hole is complete, i.e., that it is not just a blind hole that is produced, and, for another, that the laser pulses are not continued once a bore hole has been completed and possibly damage the wall regions located behind it.
For this reason, various methods for automated piercing and diameter detection have already been proposed, which infer the piercing instant and bore-hole diameter in a variety of manners on the basis of changes in specific features of the process radiation during pulsed laser drilling, cf., German Published Patent Application No. 38 35 980. However, drilling errors may still occur even when using such checking methods, which cannot be tolerated given the high quality standards prevailing in the aerospace field, in particular.